It is well known to those skilled in the art that grains having a grain diameter which is much greater than their grain thickness, i.e., so-called tabular grains, are desirable to increase the sensitivity of a silver halide photographic emulsion and further to increase sharpness, granularity, color sensitization efficiency, covering power in conjunction with a sensitizing dye, and so forth.
Furthermore, if the silver chloride content is increased, water-solubility is increased and development and fixing are achieved in a shorter time, leading to the production of silver halide suitable for rapid processing.
Silver halide grains having a high silver chloride content (hereinafter referred to as "high silver chloride content grains") generally tend to formed as cubic grains. Thus some special techniques are needed to produce such as tabular grains. In connection with high silver chloride content tabular grains having a silver chloride content of more than 50 mol%, only two methods have been known. One of the methods is described in U.S. Pat. No. 4,399,215 in which grain formation is performed using ammonia with no introduction of bromide and iodide in the inside of grain and while maintaining the pAg within the range of 6.5 to 10 and the pH within the range of from 8 to 10, and the other is described in U.S. Pat. No. 4,400,463 in which grain formation is performed in the presence of aminoazaindene and a peptizer having a thioether bond.
The methods described in the above U.S. Patents directed the preparation of silver chloride tabular grains having a high aspect ratio and large grain size, which can be easily understood from their examples. An emulsion having a high aspect ratio and a large grain size is advantageous with respect to the sensitization because the amount of a spectral sensitizing dye to be adsorbed per grain can be increased, but such is not always preferred as an emulsion for rapid developing processing, which is aimed at in the present invention. Furthermore, an emulsion having a high aspect ratio and a large grain size has serious disadvantages in handling, such as pressure marks and pressure desensitization which are characteristic of tabular grains. Thus, the emulsion is not always preferred from a practical standpoint.
Under the above circumstances, it has been desired to develop high silver chloride content grains which possess, as well as the fundamental properties that sensitivity is high and fog is reduced, suitability for rapid developing processing, and also better satisfy the practical requirements that granularity is good, pressure resistance of the grains is good, and so forth.